Spending on soft drinks dropped 3.3% between late November and a year ago, according to sales data gathered by Nielsen.

The decline in diet beverages was even steeper: Year-over-year, consumers spent 7.2% less on diet cola and 7.8% less on diet lemon-lime drinks.

It's a concern on the mind of Pepsico(PEP) CEO Indra Nooyi, who told investors in October that her company has seen "a fundamental shift in consumer habits and behaviors," adding that "the diet slowdown has been a little more rapid than we expected."

Industry watchers say the trend began about 10 years ago and is primarily driven by increasingly health-conscious consumers looking for ways to cut back on sugar and sugar substitutes.

The sales decline intensified in the last five years, said Hester Jeon, an industry research analyst at research firm IBISWorld whose study on soda production was published last month.

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Americans are on track this year to drink 3.5 gallons per person less than they did five years ago, when they drank just over 42.1 gallons of soda each year, she said.

Their response: Introduce low-calorie sodas and expand the range of products to include ready-to-drink tea, juices and sports drinks.

"Our member companies offer a wide variety of product choices, portion sizes and calorie counts, and continue to innovate with more choices to meet evolving consumer needs," said Maureen Beach, spokeswoman for the American Beverage Association, an industry group.

Industry giants Coca-Cola(CCE) and Pepsi -- which together make up about 70% of the soft drink industry, according to IBISWorld -- did not comment for this article. Pepsi referred questions to the American Beverage Association.

Nooyi, the Pepsico executive, told investors in October that her company would respond to the changing preferences with new products in 2014.

"They don't mind some calories, but they want natural sweeteners," she said. "They want to go back to sugar in some cases."